Two-center two-electron excitations identified in NEXAFS for solid Ne

Abstract

REPORT DOCUMENTATION PAGEForm Aoved PIO( t O'l'ft l b..dtn tfo tlhsi cQIIeCOn 0 flIO rmatO,|o-estin*atro to a (verge I hour tponje• including the timc ot recviewing in t.uction. seta(Ching exq sinst gt dA a $Out(,, g14ther'*n O .14 antn~ n the data needed. And Cinplt elmtg and tev.€i *ng the QI1CCUen or i* .taon. S cnd (onment$ Aiedgdrnq this burden cisrt imt, 0 any other jS•p(t O0 th,.s collec'.o. of nIlor. At.On. nc:lud-mg S.ggetiOns 10, r ,educing this b cden. Experimental K-edge NEXAFS (near edge X-ray absorption fine structure) data for solid Ne are compared with theoretical results. Seven features in the experimental spectrum that are not reproduced by the theoretical results are found to be attributable to two-electron excitations. Five of them are usual one-center excitations, but the remaining two are found to be novel two-center excitations involving resonant orbitals as opposed to the usual bound orbitals. In contrast, twocenter, two-electron excitations involving only bound orbitals are found to be absent in the experimental spectrum.PACS numbers: 78.70.Dm, 32.30.Rj, 71.90.+qIn photo-absorption spectra, one-electron (l-e) excitations normally dominate the spectra for obvious reasons. Much weaker two-electron (2-e) shakeup excitations, nevertheless, have been observed in many atomic, molecular, and solid systems. In solids and molecules, these 2-e excitations can be either one-center ( Rehr and co-workers [6,7]. The input data for these calculations include the coordinates for 87 atoms (which form 6 shells with an fcc lattice constant of 4.463A [8]), the sample temperature (6.3K), the Debye temperature (66.0K) [9], and a choice for the self-energy as 3 described below. All other information, including the core lifetime, are internally provided by the code. We have performed full MS calculations incorporating the maximum number of paths that the code allows. In Ne solid, the inelastic mean free path of the photoelectrons is very long (>80,k) for electron energies less than 20eV but rapidly decreases to 20A above 30eV [10]. In order to include this large variation in the mean free path, we have performed two calculations, one without inelastic loss [theory (a)] and one with inelastic loss [theory (b)]. We employed the ground-state, exchange-correlation potential with a zero imaginary part for (a) and the complex Hedin-Lundquist self-energy for (b). Thus, the theoretical result (a) simulates the experimental spectrum below 30eV and (b), above 30eV. These sections of the spectra are indicated by solid lines in Fig. I and represent our theoretical spectrum.The sharpest peak (El) in the experimental solid Ne spectrum closely resembles the Is -+ 3 p excitation peak for gaseous Ne, so that peak E 1 has been attributed to the ls--*3pexciton [5]. The FEFF6 model is limited to an "extended" continuum spectrum above the Below we shall discuss the origins of those Di features.The structure comprising the D 3 -D 5 features is virtually identical to the structure observed in the NEXAFS of gaseous Ne [11] except for...

Quantitative Interpretation of K-Edge NEXAFS Data for Various Nickel Hydroxides and the Charged Nickel Electrode

“…Recent work 19 also shows that the Ni atoms primarily have actual charge of +2 for all oxidation states. We utilized the Hedin−Lundquest potential, which we find, consistent with previous experience, − gives the best agreement with experiment. The maximum path length was set to 8−10 Å for the one- and two-shell calculations and to 15 Å for the nine-shell calculations.…”

“…We have previously 20-22 performed FEFF6 calculations on alkali halides and condensed rare gases (particularly Ne) to test the validity of this code and found that the code reproduces almost all of the experimentally observed peaks except the highly localized excitonic peaks very near or below the edge. For the first-row transition metals and lighter elements, the absolute energy must be shifted by 0−5 eV for optimum agreement with experiment. − Here, the FEFF6 results were optimally aligned with the experimental data for Ni(OH) 2 , and then the FEFF6 results for NiOOH and BaNiO 3 were shifted by the identical amount.…”

“…In this work we use curved-wave multiple scattering calculations on the appropriate atomic clusters to interpret the NEXAFS spectral line shapes for β-Ni(OH) 2 , β-NiOOH, BaNiO 3 , and the charged electrode (CE). We and others have shown that a detailed interpretation of NEXAFS data allows small distortions from octahedral or tetrahedral symmetry to be determined. − In the Ni hydroxides, these distortions are mainly interpreted as arising from the differences between the Ni−O and Ni−OH bond lengths. Thus the Ni−O bond is elongated to 2.07 Å when a H atom in the gallery is bonded to the O atom, as opposed to 1.88 Å when the O atom is not bonded to a H atom in the gallery.…”

Al K-Edge Near-Edge X-ray Absorption Fine Structure (NEXAFS) Study on the Coordination Structure of Aluminum in Minerals and Y Zeolites

“…Curved-wave−multiple-scattering (CW−MS) cluster calculations utilizing the FEFF6 code developed by Rehr and Albers , were performed in this work. We have previously , performed FEFF6 calculations on alkali halides and condensed rare gases to test the validity of this code and found that the code reproduces almost all of the experimentally observed peaks except the highly localized excitonic peaks very near or below the edge (particularly for Ne). For the first row transition metals and lighter elements, the absolute energy must be shifted by 0−5 eV for optimum agreement with experimentally observed results.…”

The application of synchrotron radiation and in particular X-ray absorption spectroscopy to matrix isolated species

“…Pure rare gas matrices 10.1.1. Solid Neon A number of workers have used X-ray absorption spectroscopy to study solid neon, with the emphasis on understanding the one and two electron processes occurring in the NEXAFS/XANES region [220][221][222][223][224][225][226], rather than the structural information in the EXAFS part of the spectrum. Complementary resonant X-ray absorption experiments of neon clusters with 15-4000 atoms in molecular beams have been carried out [227,228] as have calculations using improved coupled cluster techniques [229].…”